Think about better message passing framework

Right now `messages` is either an attribute of API classes (filled in dynamically over its lifespan) or a computed field (if a container of other API classes)

The ones with computed fields often do require their own class-specific messages too, currently handled in multiple places via private non-official internal fields attached after model initialization

We needed this feature for the BIDS Dandisets board to continue chugging along but open to proposals for long-term improvements that can be patched in modularly (with many tests too hopefully)

@candleindark if you wanted to think about this on the backburner